90 civic si w/ b16a1

Hey guys (and ladies), looking for some opinions. I'm strolling a 1990 honda civic si, with a moreless stock b16a1 with only typical bolt ons. Nothing too crazy, typical daily driver. My ecu would occasionally throw a engine code and when i turned the car off it'd be gone, now all of a sudden the check engine like is on all the time. I checked the ecu codes and its throwing code 9, and 23. cyp sensor, and knock sensor. Where would you guys start in order to diag this problem? Cars running like a champ as always, just concerned. Any help is appreicated..